Just because it is in the public domain doesn't mean it cannot be patented. People have patented that which is in the public domain before, are probably doing it right now and will do it in the future.
Some do it because they "re-invent the wheel" and don't know about the public domain stuff.
Some do it because they know about the public domain stuff and think it is a great idea they want exclusively for themselves.
Either way, "public domain" doesn't really protect against patents.
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